The present invention relates generally to a combustion chamber, particularly to a gas turbine engine combustion chamber.
In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxide of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and all the combustion air before combustion occurs. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection. Our International patent application no. WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly.
The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends, and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has two coaxial radial flow swirlers which supply a mixture of fuel and air into a primary combustion zone. An annular secondary fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture of fuel and air into a secondary combustion zone.
One problem associated with gas turbine engines is caused by pressure fluctuations in the air, or gas, flow through the gas turbine engine. Pressure fluctuations in the air, or gas, flow through the gas turbine engine may lead to severe damage, or failure, of components if the frequency of the pressure fluctuations coincides with the natural frequency of a vibration mode of one or more of the components. These pressure fluctuations may be amplified by the combustion process and under adverse conditions a resonant frequency may achieve sufficient amplitude to cause severe damage to the combustion chamber and the gas turbine engine.
It has been found that gas turbine engines which have lean combustion are particularly susceptible to this problem. Furthermore it has been found that as gas turbine engines which have lean combustion reduce emissions to lower levels by achieving more uniform mixing of the fuel and the air, the amplitude of the resonant frequency becomes greater. It is believed that the amplification of the pressure fluctuations in the combustion chamber occurs because the heat released by the burning of the fuel occurs at a position in the combustion chamber which corresponds to an antinode, or pressure peak, in the pressure fluctuations.
Accordingly the present invention seeks to provide a combustion chamber which reduces or minimises the above mentioned problem.
Accordingly the present invention provides a gas turbine engine combustion chamber comprising at least one combustion zone being defined by at least one peripheral wall, at least one fuel and air mixing duct for supplying air and fuel respectively into the combustion zone, the at least one fuel and air mixing duct having at least one first means at its downstream end to supply air and fuel into the at least one combustion zone at a first position in the at least one combustion zone and at least one second means at its downstream end to supply air and fuel into the at least one combustion zone at a second position in the at least one combustion zone, wherein the second position is downstream from the first position to increase the distribution of fuel and air discharged from the fuel and air mixing duct into the combustion zone to increase the distribution of heat released from the combustion process whereby the amplitude of the pressure fluctuation is reduced.
Preferably the distance between the first and second positions is substantially equal to the velocity of gas flow multiplied by half of the time period of one cycle of the pressure fluctuation of a predetermined frequency to reduce the amplitude of the pressure fluctuation at the predetermined frequency.
The combustion chamber may comprise a primary combustion zone and a secondary combustion zone downstream of the primary combustion zone.
The combustion chamber may comprise a primary combustion zone, a secondary combustion zone downstream of the primary combustion zone and a tertiary combustion zone downstream of the secondary combustion zone.
Preferably the at least one fuel and air mixing duct supplies fuel and air into the secondary combustion zone.
The at least one fuel and air mixing duct may supply fuel and air into the tertiary combustion zone.
The at least one fuel and air mixing duct may supply fuel and air into the primary combustion zone.
The at least one fuel and air mixing duct may comprise a plurality of fuel and air mixing ducts.
Preferably the at least one fuel and air mixing duct comprises a single annular fuel and air mixing duct.
The at least one fuel and air mixing duct may have at least one third means at its downstream end to supply air and fuel into the at least one combustion zone at a third position in the at least one combustion zone, wherein the third position is downstream of the first position and upstream of the second position.
The at least one fuel and air mixing duct may have at least one fourth means at its downstream end to supply air and fuel into the at least one combustion zone at a fourth position in the at least one combustion zone, wherein the fourth position is downstream of the third position and upstream of the second position.
The at least one fuel and air mixing duct may have at least one fifth means at its downstream end to supply air and fuel into the at least one combustion zone at a fifth position in the at least one combustion zone, wherein the fifth position is downstream from the fourth position and upstream of the second position.
The first means may direct the fuel and air mixture into the at least one combustion zone at an angle of 50xc2x0 and the third means directs the fuel and air mixture into the at least one combustion zone at an angle of 30xc2x0.
The first means and the second means may be arranged alternately around the peripheral wall.
The first means may direct the fuel and air mixture into the at least one combustion zone at an angle of 55xc2x0 and the third means directs the fuel and air mixture into the at least one combustion zone at an angle of 45xc2x0, the fourth means directs the fuel and air mixture into the at least one combustion zone at an angle of 35xc2x0 and the second means directs the fuel and air mixture into the at least one combustion zone at an angle of 25xc2x0.
The first means may direct the fuel and air mixture into the at least one combustion zone at an angle of 50xc2x0 and the third means directs the fuel and air mixture into the at least one combustion zone at an angle of 45xc2x0, the fourth means directs the fuel and air mixture into the at least one combustion zone at an angle of 40xc2x0, the fifth means directs the fuel and air mixture into the at least one combustion zone at an angle of 35xc2x0 and the second means directs the fuel and air mixture into the at least one combustion zone at an angle of 30xc2x0.
The first means, second means and third means may be arranged alternately around the peripheral wall.
The first means, the second means, the third means and the fourth means may be arranged alternately around the peripheral wall.
The first means, the second means, the third means, the fourth means and the fifth means may be arranged alternately around the peripheral wall.